Conventionally, to impart novel characteristics to characteristics inherent to rubbers, introduction of a functional group into a rubber molecule has been known. For example, Patent Document 1 describes an isoprene oligomer formed from a trans structural moiety and a cis structural moiety, the isoprene oligomer being represented by a particular formula; at least one atom or atomic group contained in the trans structural moiety is substituted by another atom or atomic group; as well as a polyisoprene formed from a trans structural moiety and a cis structural moiety, the polyisoprene being represented by a particular formula; at least one atom or atomic group contained in the trans structural moiety is substituted by another atom or atomic group. In Patent Document 1, the isoprene oligomer or the polyisoprene bonds to a phosphate at its terminal. The carbon atom at the terminal of the isoprene oligomer or the polyisoprene bonds to a phosphorus atom via an oxygen atom.